The present invention relates to novel solid polymeric electrolytes (SPEs) and to electrochemical assemblies fitted with such solid polymeric electrolytes.
Electrochemical assemblies fitted with aqueous electrolytes are well known. Examples of such assemblies comprise conventional Leclanchxc3xa9 cells and Nixe2x80x94Cd, Nixe2x80x94Zn, Znxe2x80x94MnO2, Nixe2x80x94H2 and Pb acid cells. These assemblies typically comprise a positive electrode (cathode), a negative electrode (anode), a liquid aqueous electrolyte and a solid separator. The separator is present to prevent any discharging which might be brought about by contact between the two electrodes.
It is known that the presence of a separator entails certain drawbacks such as possible chemical incompatibility with the electrolyte, resulting in a reduction in the lifetime of the batteries, a decrease in the impedance of the electrochemical cell and the complexity of implementation.
Accordingly, it has for many years been proposed to replace liquid electrolytes with solid polymeric electrolytes, which have the advantage of dispensing with the separator.
Many studies have been carried out in this respect.
Thus, an electrolyte of this type in which the matrix comprises, partly or totally, homo- and/or copolymers essentially containing non-crosslinked chains, derived from one or more monomers comprising a hetero atom capable of forming bonds of donor-acceptor type with the cation of the ionizable salt, is known from French patent No. 2,442,513.
However, the complex formed has a strong tendency to crystallize (reduction of the lability between chains), at a temperature below the melting point of the crystallites (generally at about 60xc2x0 C.), which appreciably reduces the ionic mobility inside the network itself. Moreover, it cannot be easily cross-linked to prevent creep.
The consequence of this crystallization is an appreciable reduction in ionic conductivity, this reduction thus making it necessary to use these solid polymeric electrolytes above the melting point of the crystalline zones in order to obtain sufficient ionic conductivity.
It is also known, in particular from European patent application No. 0,037,776, to use crosslinked, in particular chemically crosslinked, polymers or oligomers to make the complexing polymer, these polymers or oligomers having a glass transition temperature which is as low as possible.
However, such polymers prove to be relatively unusable in the context of their applications at room temperature as solid electrolytes in electrochemical power sources on account of their low ionic conductivity and the instability of the crosslinking bonds with respect to the electrodes, in particular the negative electrode; this is particularly the case for di-, tri- or multiisocyanate chemical crosslinking bonds with respect to lithium which reduces them.
European patent application No. 0,078,505 describes a solid polymeric electrolyte formed from an ionizable salt, a complexing polymer and a polymer which is miscible with the said complexing polymer, and which is crosslinked, the complexing polymer being maintained in an essentially amorphous state during this crosslinking operation.
The complexing polymer is especially a polyether. The miscible polymer is especially an elastomer.
European patent application No. 0,119,912 describes a copolymer of ethylene oxide and of a second monomer unit chosen from substituted cyclic oxide ethers.
Other solid polymeric electrolytes have also been proposed.
U.S. Pat. No. 5,030,527 describes a polymer which is crosslinkable by radiation, such as acrylic polymers, vinyl alcohol polymers, epoxy polymers and polyimines.
European patent No. 0,657,484 describes a copolyether which can be crosslinked by radical means, comprising ethylenic substituents, which can be used for the preparation of a solid electrolyte.
U.S. Pat. No. 3,969,147 describes a solid polymeric electrolyte formed from a mixture of starch and crosslinked starch.
U.S. Pat. No. 5,424,150 describes a solid polymeric electrolyte formed from a copolymer of ethylene and of a saponified monomer ester of vinyl alcohol onto which a polyoxyethylene has been grafted.
The abstract from Japanese patent JO 2,235,957 describes a non-crosslinked solid electrolyte formed from an epichlorohydrin-ethylene oxide copolymer and from an alkali metal salt.
The abstract from Japanese patent JO 4,145,145 describes a liquid electrolyte impregnated into a rubber of the epichlorohydrin type.